She exhaled, slipped off her underwear and draped a towel over the lower half of her body.

Minutes later, Eric Amaranth, a gangly and unassuming man in a blue T-shirt, emerged from the bathroom. He had just cleaned and put condoms on the eight sex toys he had brought along from his Gramercy Park studio.

Susan took one — a Mystic Wand vibrator. And for the next two hours on a hot July evening, at the bargain price of $240 (plus the cost of one night at the Hilton in Midtown), Susan lay back as Amaranth stood in a corner and watched her masturbate.

“[The session] was a chance for me to sort of take control of my sexuality,” says Susan, a 35-year-old analyst at a wealth management firm in Fairfield County. She comes from a “very strict Protestant household,” has had eight sexual partners total and got out of her last serious relationship two years ago. She asked that we use a fake name.

“I realize now that I’m sure I played a part in having a sex life that wasn’t so satisfying,” Susan says, adding that all her initial nervousness about Amaranth “melted away.”

“He has a way about him,” she says.

One would hope so — he is a “sex-life coach” after all. Since 2010, clients have been paying New York-based Amaranth to provide them with getting-down guidance.

For $175 an hour, you can have a “talk session” with Amaranth, where he acts as a therapist of sorts.

Or for $240 for two hours, you can upgrade to a “guided session,” where he will watch you pleasure yourself and/or you and your partner have sex — providing tips and tools to improve the experience along the way.

But the so-called “sex samurai” doesn’t ever participate or touch. That, he explains, would be prostitution.

“I seem to have a talent for brainstorming sex stuff and coming up with really great material,” he says.

His client list has been on the rise — he now has 35, evenly split by gender. Ninety-percent opt for titilating talk-only.

“I have a lot of New York clients who say, I want to be really good at this because I’m really good at everything,” explains Amaranth, who says he has slept with approximately 65 women (come on — you wanted to know).

To help his pupils reach sexual perfection, they need to iron out a few kinks first, including issues like: achieving better orgasms (or achieving orgasm at all), pleasing partners, coital communication, early emission, bedroom boredom and body image.

Kate, a 37-year-old bisexual who lives on the Upper East Side, decided to see Amaranth because, she admits, her past performances “could’ve been better,” especially with men.

“I’m interested in excellence in pretty much every facet of my life,” says Kate, a guitarist, who asked that only her first name be used. “So if there’s a way I can be better, do better, I’m willing to explore that.”

She shelled out $300 for a double-decker talk sesh, and just purchased another four-pack for $540. During their first Skype session, they used up all 60 minutes dissecting the art of the hand job.

“It’s like getting a teacher in any subject, it just gets you to the finish line faster,” she says, justifying the cost.

“After a certain age, it became uncool to admit, I don’t really know what I’m doing,” says Michael, a 40-year-old educator from Park Slope, who is another acolyte of Amaranth.

Eight sessions this year and $1,000-plus in the hole, Michael keeps coming back for more. And so do his conquests. Especially now that he’s mastered the ever-mystical “A-spot, because that’s gotten the biggest reaction,” he says of the erogenous zone allegedly located between the G-spot and the cervix.

So just what makes Amaranth a self-proclaimed sexpert?

“Instead of taking an academic approach to credentials, [I’ve taken] an apprenticeship approach,” he explains.

After graduating from William & Mary with a degree in English in 1999, Amaranth, 35, decided to follow his “pipe dream” of being a sex life coach.

Armed with bookloads of knowledge (“I had a lot of info comparatively for someone my age because I did a lot of sex self-help book reading,” Amaranth boasts), he met with Betty Dodson, a well-known sex educator 48 years his senior, to chat about copulation.

“I could tell from his e-mails that he was very unusual,” says Dodson, 83. “Although he was only 22, he was more sophisticated than many men my age.”

“We spent the weekend together and had sex and it was just fabulous. He was a natural,” she says.

And he was in luck.

“I was looking to have a young man as an apprentice,” recalls Dodson.

Amaranth ended up shacking up with his mentor-turned-lover for 10 years in Dodson’s Manhattan apartment. He struck out on his own about two years ago.

It was about that time that he met his now 28-year-old girlfriend after she “won” him during a benefit auction at the Museum of Sex. During their session, the teacher-student dynamic started to shift.

“We were really hitting it off and since she never paid me for anything I said, ‘I’m feeling a moment here. Would you like to continue on a professional format or would you like to get personal?’ ”

The two have been dating on and off ever since. “She’s very pro me helping people with this,” he says about the career he intends to continue for as long as clients will have him.

“He’s very righteous. He’s very dedicated. He really wants to have sex be better for people,” says Dodson. “It’s not a gimmick thing for him. He’s very serious.”

Earnest or not, sex-life coaching, especially of the “live” variety, makes some people squirm — and not in a good way.

“Could this be a viable way to help some couples? Sure. Is it right for everyone? Definitely not.”

“It’s not necessary to be coached in real-time by an observer in order to improve your sexual technique,” says Kerner. “To equate sexual intimacy with sports mastery is an apples-to-oranges comparison.”

But Amaranth’s clients are insatiable.

Kate, who has only done talk sessions up until this point, is giddy about having her first “live” session with Amaranth.

“He’s like, ‘Don’t worry.’ He’s super reassuring. He said his experience has been, once you’re there and things get hot and heavy, you sort of forget he’s there,” says Kate.

She might want to heed Susan’s advice to double up on class-time.

“It went by!” Susan says of her time with Amaranth.

“Toward the end, I was like, oh, can we do blank, blank and blank . . . I had a list of things.”

“I would actually urge people to consider the four hours,” she advises, “because after the two hours you’re like, ‘Teacher, tell me more!’ ”